You can have bad sound and out of focus images, but if the audience is cheering and clapping when the credits roll you've succeeded.
Ah... While this statement appears entirely logical, it is in practise a non-sequitur logical fallacy, a quite common fallacy amongst indie filmmakers. If the sound fails to meet the technical standards required by the festival/distributor/broadcaster, regardless of how good the film is overall, the audience will not cheer and clap when the end credits roll because there will be NO audience, not because they all "walk out", but because they cannot "walk in" in the first place!
So it's settled, sound must be the most important aspect of filmmaking. Well, no! You can have great sound but if the legalities; copyrights, clearances, etc., are not in order, your film cannot be distributed and again, no audience! But, even with great sound, sorted legalities and therefore a distributable film, without marketing no one will be enticed to pay to see the film. Again, no audience!
I mention this simply because many indie filmmakers are essentially hobbyists, even though they may aspire to be something else. I don't mean this in the derogatory sense, I mean they tend to make films primarily for themselves, focusing on their own filmmaking interests, and in so doing, avoid (as much as possible) those aspects of filmmaking they consider to be uncreative/uninteresting. Fully understanding market forces/expectations AND technical demands (and applying that understanding) is both uncreative and uninteresting to the vast majority of filmmakers and is therefore the most fundamental difference between hobbyist and professional filmmakers. Fortunately, platforms like YouTube and provincial film festivals have virtually no minimum technical standards or market expectations and therefore provide an outlet for hobbyist filmmakers.
The OP, and most subsequent contributors to this thread, don't appear to be talking in hobbyist films terms but in professional/commercial film terms or at least, potentially commercially viable film terms. In which case, the first, most fundamental and therefore "MOST important aspect of filmmaking" is finding out what the commercial demands/expectations are and identifying the resources which will realistically enable those expectations and demands to be met. All the other aspects of filmmaking, the aspects which determine if a film is actually any good or not, ultimately require people (audiences) to make that determination, which of course is impossible if they never get to see it!
G